One type of axial piston hydraulic unit conventionally employs a rotary cylinder block including a plurality of axial bores or cylinders having a plurality of axially disposed pistons reciprocable in the block. A relatively stationary valve member having inlet and outlet ports engages one end of the rotating cylinder block so that the cylinders in the block serially communicate with the inlet and outlet ports as the block rotates. In such devices the pistons may have spherical ends carrying pivotal slippers which engage an angular cam or swashplate so that the pistons reciprocate in the cylinder block.
When the hydraulic unit acts as a pump, the block is rotated and fluid is drawn into the cylinder through the inlet port as the pistons withdraw away therefrom the bores. Piston return mechanisms are usually provided for withdrawing the pistons through the intake stroke and for maintaining engagement between the slippers and the cam. As the pistons pass over top dead center they begin movement into the cylinders, discharging high pressure fluid through the outlet ports in the valve member. When the device operates as a motor, the reverse operation occurs, with high pressure fluid entering the inlet port forcing the pistons out of the cylinders thereby effecting rotation of the cylinder block.
In recent years hydraulic component applications in various industries have become increasingly taxing. For example axial piston pumps and motors are being required to far exceed their original design capabilities and increases in both hydraulic pressure and rotational speeds are causing higher rates of failure in axial piston pumps and motors.
One of the areas which is more failure prone is the interface between the rotating barrel or cylinder block and the valve member. A good seal is required between the two, and to keep such a seal, two requirements must be met: first, the mating surfaces must be extremely flat and perfectly parallel and secondly, proper axial alignment between the barrel or cylinder block and the valve member must be maintained. If the two are slightly axially misaligned i.e. relative to one another, increased wear of the mating surfaces on the cylinder block and valve member will occur, leading to premature failure. Also misalignment may cause excessive leakage, which effects efficiency and disturbs the hydrostatic balance of the cylinder block.
In the areas of the porting faces formed by the cylinder block, the porting faces being that area of the cylinder block which abuts the valve member so as to provide a passage between the cylinders and the inlet and outlet ports of the valve member, varying types of bearing materials, such as bronzes, are usually employed. Bearing materials are those having favorable characteristics in the categories of conformability, embedability, wear resistance, abrading tendency and corrosion resistance. The bearing material provides, ultimately, sliding friction characteristics favorable for operation.
In some applications, the entire block is made out of a bearing material while others use a composite of steels in the high stress areas and bearing material in the bearing areas such as in block bores and the porting faces. Common to all of these is the fact that a bearing type material is normally used on the port face. However, use of the bearing type material provides problems in that a too soft bearing material tends to erode as a result of cavitation at or around the porting face. Further coupled with this problem is the fact that a misalignment may occur due to the cavitation caused erosion, thus increasing the chance of unit breakdown. Erosion and cavitation also adversely effects the balance and efficiency of a unit.
The present invention is directed to providing an axial hydraulic displacement unit which overcomes one or more of the above problems.